In recent years, there has been major expansion in optical fiber transmission systems and it will continue in years to come, particularly in the field of telecommunications, because of the immense potential of optical fiber transmission systems in comparison with systems based on copper. For wideband service applications in distributed or switched networks, the use of optical fibers today appears technically inescapable and economically viable because of the ever decreasing cost of manufacturing optical fibers. For switched services, authorizing the interchange of information between two subscriber data terminals (visiophones), and for distributed services, where the subscriber terminal is a passive termination that does not interact with the distribution center (distributing television programmes), networks have resorted to optical fiber cables defining multiple subdivisions, or ramifications, in order to reach subscribers.
FIGS. 1 and 2 respectively show two known cable structures of the prior art. In a cross-section of the cable, an outer sheath 1 and a plurality of optical fiber modules referenced 30 to 36 can further be distinguished. Said modules define separate ducts which can be identified with a view to facilitating subscriber connections, for example, in the vicinity of the terminations of the network. Each of said modules 30-36 is in the form of a module sheath surrounding a respective group of optical fibers. In the variant of FIG. 1, the outer sheath 1 of the cable comprises two concentric sheath elements 10 and 12, respectively an inner element and an outer element, and a reinforcing sheath 13 disposed between the two sheath elements. In a variant, the reinforcing sheath 13 could be replaced by at least two longitudinal reinforcing elements uniformly distributed over the periphery of the inner sheath element 13. Sealing threads or ribbons 20, 21, 22, 23, and 24 are further provided, disposed longitudinally inside the cable. In the variant of FIG. 2, a filler material 40 fills the volume inside the outer sheath that is not occupied by modules 30-36. The invention bears on the characteristics of an optical fiber module and not on those of a cable structure, so the variants relating to the production of such a structure are not given in greater detail in the following description, but the person skilled in the art will understand that the outer sheath can take various forms, such as metal or plastics, and that modifications, additions or deletions of a known type can be applied to the structures described in FIGS. 1 and 2.
The prior art, such as recalled in European patent application EP-A-0 468 878, describes optical fiber modules having a thin module sheath in order to make them easy to tear. Each of the optical fiber modules is obtained by extruding a plastics material, such as polyethylene or polypropylene, around a group of optical fibers, in order to form the module sheath. A first drawback of such a module is that it requires an extrusion operation in order to manufacture it. A second drawback is that the quantity of material required for forming the module sheath, although considered as small specifically because of the thinness of the module sheath, nevertheless remains relatively high, with the result that the manufacturing cost of a cable is increased.
The invention seeks to remedy the above-mentioned drawbacks by providing, in particular, a method of obtaining an optical fiber module that does not require any extrusion operation. The invention further seeks to substantially reduce the quantity of material needed for forming the module sheath in comparison with known embodiments. As a consequence of the above, the invention enables the density of optical fibers in a cable to be increased. Furthermore, the invention provides a particularly advantageous optical fiber cable.
To this end, according to the invention, a method of forming a sheath for an optical fiber module, with the module being designed to form part of an optical fiber cable including an outer sheath enclosing at least two optical fiber modules each including a respective group of optical fibers, comprises the steps of:
deforming a film around said group of optical fibers; and
fixing the film thus deformed around the group of optical fibers in order to surround and hold said group of optical fibers.
The deforming step, as contemplated in the present invention, encompasses tightly wrapping a pre-formed film around a group of optical fibers and excludes an extrusion procedure for encasing the fibers. The deforming step is typically preceded by the step of:
applying traction to the group of optical fibers.
In a first variant, the deforming step consists in deforming the film widthwise around the group of optical fibers, which runs under the applied traction so that a first face of said film overlaps a second face of said film over a portion of given width of the film.
In a second variant, the deforming step consists in helically taping said film around the group of optical fibers which runs under the applied traction.
The step of fixing the film deformed around the group of optical fibers is typically obtained by adhesive.
Furthermore, the film can be a heat-shrink material, in which case, the fixing step is followed by a step of heating the film so that it shrinks around the group of optical fibers.
The invention also provides a cable containing the optical fiber modules. According to the invention, in such an optical fiber cable, including an outer sheath containing a plurality of optical fibers separated into at least two optical fiber modules, with each of said modules consisting of a module sheath surrounding a respective group of optical fibers, the module sheath is formed by a film made of plastics material.
In a first variant of the invention, the film is wrapped widthwise around a group of optical fibers, a first face of said film being superposed on and fixed to a second face of said film over a portion of given width of said film. For example, the first face of the film is fixed by adhesion to the second face of the film over said portion of given width.
In a second variant, the film is taped helically around the group of optical fibers.
A film of the invention is not limited to forming one module, but can be used to constitute complex modules, i.e. groups of modules. To this end, according to the invention, an optical fiber cable including an outer sheath enclosing a plurality of optical fibers separated into at least two optical fiber modules, with each of said modules consisting of a module sheath surrounding a respective group of optical fibers, has at least one film bringing together at least two of said modules.
Alternatively, the film is wrapped widthwise around said at least two of the modules, a first face of the film being superposed on and fixed to a second face of the film over a portion of given width of the film, or the film is taped helically around said at least two of said modules.
A cable of the invention can have module identification information on an outside face of the film.
Inside the outer sheath of the cable, each of the modules can be disposed either substantially rectilinearly, or substantially helically, or even substantially in SZ manner.
The invention also envisions a complex optical fiber cable that includes at least two cables of the type described above.